A simple and highl y sensitive fiber in-line Mach-Zehnder interferometer (MZI) temperature sensor is demonstrated,its responses to surrounding refractive index and temperature are analyzed theoretically and expe rimentally,and especially the different mechanisms of the fiber MZI before and after sealing are compared for temperature sensing.The interferometer is realized by splicing three t elecom single-mode fibers with large “overlap” splicing parameters to form two waist-enlarged fusion bitaper s.Experimental results show that the MZI is sensitive to surrounding refractive index (RI).Then the MZ I is sealed in a capillary tube filled with Cargille oil.Due to the thermo-optic effect of sealing-liqui d and the intrinsic high RI sensitivity of the interferometer,the sensor produces a very high temperature s ensitivity of 150pm/℃ within the range of 10－40℃.The sensitivity is double times of that of the bar e MZI in air and it is much higher than that of some fiber grating-based sensors or fiber in-line interferometric sensors.Moreover,the proposed sensor has advantages of low cost,easy-to-fabrication and good physical stren gth,which makes it promising in environmental monitoring,food industry,and other fields for temperature sensing.